Product quality or color control is an essential quality criterion in many technical on-line processes. The production of plastic containers, in particular plastic bottles for the beverage industry, using granulated recycled material, requires separation and sorting of different color portions, different plastic types, e.g. polyethylene, polyamide, polyvynylchloride, the recognition and sorting of e.g. fractions contaminated with gasoline, diesel, benzene, toluene, xylene, to enable re-use e.g. in the food industry. To achieve this object, the reusable pure base materials must be differentiated and separated from the contaminated granulates and moreover, the fragments having different colors must be sorted to obtain portions of pure color.
Prior art methods and devices unsatisfactorily achieve this complex object to a limited extent only. Devices are known which permit recognition of color with subsequent color sorting by controlled air flow nozzles and using CCD color cameras. These systems are disadvantageous in that the measuring speed is unacceptably slow and the signal-to-noise ratios too low, and are therefore not suitable for use in the present technical process. Moreover, methods for identifying and separating different plastic materials have been developed. The methods used thereby are not suited for use with granulates or granulate-similar portions of recycled materials, since the intended identification and sorting of smaller or larger bottles, containers etc. requires 1000 times faster measuring speeds to cope with the minimum mass flow required for economic reasons. A further drawback consists in that the detection of colors and the differentiation of materials require separate systems using different physical principles, i.e. CCD cameras for detecting the color or NIR absorption for plastic classification, wherein the total cost per system is above the economically justifiable level. The largest drawback is the fact that none of the above-mentioned methods allows proof of contamination by foreign matter deposited in the base material. The latter is absolutely required e.g. within the scope of recycling plastic material used in connection with food, which is to be used a plurality of times as food containers.
It is therefore the underlying purpose and object of the present invention to propose a novel method or devices which not only enable identification of color and classification of material but also identification of contamination in the base material, wherein, for economic reasons, the three problems shall be solved simultaneously using only one single basic physical principle.
The present invention solves these three problems in that the recycling granulate, which is transported on a conveyor belt, is illuminated by a laser beam and the radiation re-emitted by the granulate is spectroscopically analyzed within a broad spectral range. The invention additionally provides means for extensive detection and evaluation of the spectra to determine the colors of the base materials and the contamination of the granulates, in real time. The invention is explained in more detail below in FIGS. 1 through 9.